Synchronous-type motors operating in field-oriented control are commonly used in applications which require high efficiency and high-fidelity speed/position control. Synchronous motors are AC motors typically including stators having multi-phase AC electromagnets which generate a rotating magnetic field. A rotor may contain permanent magnets or electromagnets which interact with the stator magnetic field to rotate at the same speed.
Field-oriented control is a method of regulating stator currents in a three-phase synchronous motor. Field-oriented control typically includes determining the motor magnetic field within three spatial axes (e.g., a, b, c axes associated with the respective stator electromagnets) and projecting these magnetic fields onto complex two-phase axes (e.g., α, β axes) using the Clarke variable transformation. From this set of axes, the Park variable transformation may be used to project the magnetic fields onto the direct axis (d) and quadrature axis (q) which define a rotating reference frame. The rotor magnetic field ({right arrow over (B)}r) and the stator magnetic field ({right arrow over (B)}s) may be projected in this dq reference frame for modeling purposes. Notably, it is desirable to orient the stator magnetic field ({right arrow over (B)}s) 90° ahead of the rotor magnetic field ({right arrow over (B)}r) for maximum torque production. Field-oriented control is used to maintain this desired orientation.
However, field-oriented control requires accurate knowledge of the flux angle or rotor magnetic field ({right arrow over (B)}r) of the rotor to correctly orient the stator magnetic field ({right arrow over (B)}s). Additionally, a speed signal may be useful for speed control. This information could be acquired directly through some type of sensor, but for cost and reliability purposes sensorless solutions are generally preferred.
Sensorless solutions for field-oriented control typically use observers to estimate the back electromotive force (EMF) of the motor in the stationary reference frame (usually the αβ, but also the abc reference frame) and then feed these back EMF signals into a phase-locked loop (PLL) using a dq (direct-quadrature) transformation. However, these solutions suffer from the compounding of error in the αβ observed back EMFs which are combined via the dq to obtain a third error signal for the PLL. Moreover, in the αβ frame, the back EMF signals are time-varying (sinusoidal or trapezoidal depending on motor type), and thus difficult to reliably track.
Accordingly, a synchronous motor that operates using improved field-oriented control would be desirable. More particularly, a system and method for operating a synchronous motor with fewer sensors and improved torque and speed response would be particularly beneficial.